Active noise cancellation is well-known within many noise control applications, e.g. control of noise in ducts, such as attenuation of fan noise in a ventilation system. In principle, a loudspeaker is positioned in connection with the duct and supplied with an electric input signal such that the loudspeaker produces an acoustic output that essentially cancels the acoustic noise emitted in the duct. Loudspeakers capable of producing an acoustic output sufficient to cancel noise at medium frequencies can easily be found, and such loudspeakers can be fitted in the duct system, since they will typically have a reasonable size compared to the size of the duct system. Compared to passive silencers using the reflective chamber principle, the absorption principle, or the ¼, -wavelength resonator principle, active noise cancellation systems can in many applications provide advantageous solutions with respect to the required size. Especially, active noise cancellation is advantageous for cancellation of noise with significant pure tone components.
Low frequency noise, such as low frequency tonal components originating from the blade frequency of a propeller, a fan or the like, requires a high acoustic output for cancellation, e.g. in the frequency range 20-200, Hz or even lower. This is the case e.g. in air conditioning and/or ventilation duct systems with one or more large ventilation fans providing air flow in the duct system. Passive silencers to effectively attenuate such low frequency components have large dimensions, and thus active noise cancellation is an alternative. However, still high output at low frequencies requires a loudspeaker system with a large total diaphragm area, and in order to provide an acceptable electric to acoustic transfer efficiency, such loudspeaker system will require a cabinet with a large volume, such as a closed box or a bass reflex box. Such loudspeaker system is a standard component and can thus be achieved in low price versions, and the physical fitting between loudspeaker cabinet and duct to be silenced is rather simple. However, the size of such loudspeaker is a problem in many applications, since most often e.g. ventilation ducts are fitted into a rather small space and thus extra space for a large loudspeaker cabinet is difficult or even impossible to find. In principle, it is possible to obtain a large low frequency acoustic output from a loudspeaker in a small cabinet, however this requires a powerful and therefore expensive power amplifier together with a loudspeaker driver with a high power handling capacity.
U.S. Pat. No. 4,947,434, discloses an active noise cancellation system for cancelling noise in a ventilation duct with the use of a loudspeaker with a sheet-like shape and made of a piezoelectric material without any cabinet. The sheet-like loudspeaker can be bent into e.g. a tubular shape and thus fit along a wall of a tubular shaped duct, or it can be mounted on an outside wall of the duct. By the use of a large and flat loudspeaker, space is saved compared to the use of a conventional electro-dynamic loudspeaker driver. However, such flat loudspeaker driver is rather expensive since it is not a standard component, and in spite of a larger diaphragm area, it will not be able to produce large acoustic outputs at low frequencies.
JP 2006 118422, by Canon Inc. discloses an active noise reducing device for reducing noise from a ventilation fan placed inside an electronic apparatus. A loudspeaker is placed in a noise cancellation branch duct which is connected respectively to the inlet and outlet side of the ventilation fan, and by means of an active feedback system, the loudspeaker acoustically “short circuits” the noise source inside the electronic apparatus, thereby reducing fan noise transmitted to the environments by the electronic apparatus. However, such system is not intended to nor suited to attenuate noise at very low frequencies, i.e. below 200 Hz, which is the frequency range that causes the major noise problems in room ventilation systems.